This invention relates generally to thin film transistor (TFT) processes and fabrication, and more particularly, to a polycrystalline film, and method of forming the polycrystalline film, from a microcrystalline film.
The demand for smaller electronic consumer products with higher resolution displays, spurs continued research and development in the area of liquid crystal displays (LCDs). The size of LCDs can be decreased, and the performance enhanced, by incorporating the large scale integration (LSI) and very large scale integration (VLSI) driver circuits, presently on the periphery of LCDs, into the LCD itself. The elimination of externally located driving circuits and transistors will reduce product size, process complexity, a number of process steps, and ultimately the price of the product in which the LCD is mounted.
The primary component of the LCD, and the component that must be enhanced for further LCD improvements to occur, is the thin film transistor (TFT). TFTs are typically mounted on a transparent substrate such as quartz or glass. TFT performance is improved by increasing the electron mobility in the devices. Increased electron mobility results in brighter LCD screens, lower power consumption, and faster transistor response times. Many of these performance enhancement features are due to the improved switching characteristics associated with TFTs. In addition, further LCD enhancements require uniform TFT performance. That is, display and driver transistors across the entire display must operate at substantially the same level of performance.
The carrier mobility of transistors formed from amorphous silicon is poor, insufficient for LCD circuit driver circuits. The carrier mobility of transistors is improved by using crystallized silicon. Uniformity in transistor performance requires that the crystalline film, from which the TFTs are formed, include large areas of uniform crystalline structure. Preferably, the crystalline film from which the TFT semiconductors are made, is crystallized in one uniform crystallographic pattern. This uniform pattern, or single crystal formation, ensures that transistors across the film have identical performance characteristics. However, single crystal silicon films, for use with LCDs, are difficult to fabricate when adhered to relatively fragile transparent substrates. In between the superior performance of single crystal films, and the poor performance of amorphous silicon, are polycrystalline films. Typically, polycrystalline films include multiple areas of crystallization that are adjacent, but of different crystallographic orientations. That is, the film is composed of many different crystallized areas with random shapes and random crystallographic orientation. Uniform performance across a polycrystalline film is enhanced by making the areas of crystallization, or grains, as large as possible.
Large areas of uniform crystallization inside the polycrystalline film ensure uniform performance in each particular crystal grain. In addition, the performance of transistors manufactured from the polycrystalline film can be enhanced by decreasing the number of grain boundaries, or areas of intersecting between different crystal grains. The boundary areas between crystal grains form electron traps which reduce electron mobility in TFTs. As a result, the device stability is decreased as the threshold voltages and leakage currents of such devices are increased.
One problem in making polycrystalline film with large grains, and therefore improved TFTs, is the necessity of using amorphous material as a building block. Another problem is the relatively low temperatures that the glass and quartz substrates, upon which the TFTs are built, are able to withstand before degrading. Typically, the transparent substrate is covered with a film of amorphous matter such as silicon or a silicon-germanium compound. The amorphous matter is heated, or annealed, so that the amorphous material takes on a crystalline form. Typically, the annealing process is limited by the requirement that the amorphous material not be heated above a temperature of approximately 600.degree. C. Above this temperature the transparent substrates are often damaged.
Various annealing methods exist for turning amorphous silicon into polycrystalline silicon. Solid phase crystallization (SPC) is a popular method of crystallizing silicon in a furnace. In this process, amorphous silicon is exposed to heat approaching 600.degree. C. for a period of at least several hours. The heat is typically generated from a resistive heater heat source. A rapid thermal anneal (RTA) uses a higher temperature but for very short durations of time. Typically, the substrate is heated during the annealing process at a temperature between 400 and 500.degree. C.; the amorphous film and transparent substrate are mounted on a relatively low temperature heated surface, or susceptor. In this manner, the silicon is heated to a temperature in the range between 700.degree. C. and 800.degree. C. without degrading the transparent substrates upon which the film is mounted. One method of performing this anneal is using the IR rays of a heat lamp, such as a halogen heat lamp.
An excimer laser process (ELC) or excimer laser anneal (ELA) has also been used with some success in annealing amorphous silicon. The laser allows areas of the amorphous film to be exposed to very high temperatures for very short periods of time. Theoretically, this offers the possibility of annealing the amorphous silicon at its optimum temperature without degrading the transparent substrate upon which it is mounted. However, use of this method has been limited by the lack of control over some of the process steps. Typically, the aperture size of the laser is relatively small. The aperture size, power of the laser, and the thickness of the film may require multiple laser passes, or shots, to finally anneal the silicon. Since it is difficult to precisely control the laser, the multiple shots introduce non-uniformities into the annealing process.
The process of heating amorphous silicon to form crystallized silicon is not entirely understood, and research on the subject continues. Variations such as temperature, film thickness, the degree to which the amorphous matter melts, impurities in the film, and a range of other factors influence the annealing of amorphous silicon. Generally, the largest grains of crystallization occur in a polycrystalline film at a specific temperature near the melting point. Temperatures below this preferred temperature do not melt the amorphous silicon enough to form large grain areas. Temperatures above the preferred temperature rapidly lead to bulk nucleation. The bulk nucleation of amorphous matter results in relatively small grain sizes.
The method of depositing amorphous silicon on the transparent substrate is also crucial in the fabrication of polycrystalfine films having large crystal grains. Typically, the transparent substrate is mounted on a heated susceptor. The transparent substrate is exposed to gases which include elements of silicon and hydrogen. The gases decompose to leave solid phased silicon on the substrate. In a plasma-enhanced chemical vapor deposition (PECVD) system, the decomposition of source gases is assisted with the use of radio frequency (RF) energy. A low-pressure (LPCVD), or ultra-high vacuum (UHV-CVD), system pyrolytically decomposes the source gases at low pressures. In a photo-CVD system the decomposition of source gases is assisted with photon energy. In a high-density plasma CVD system high-density plasma sources, such as inductively coupled plasma and helicon sources are used. In a hot wire CVD system the production of activated hydrogen atoms leads to the decomposition of the source gases.
It would be advantageous if the grains of annealed polycrystalline film could be made larger, as large as 10 microns. Since several active devices could be built in a such a large grain, the effect would be that of a single crystal film in a localized area.
It would be advantageous if the grain sizes in a polycrystalline film could be made uniform so as to minimize the difference between grains in the film, and therefore, minimize differences between active devices in different grain areas.
It would be advantageous if the grain areas of a polycrystalline film could be fabricated to have the same crystallographic orientation to minimize the differences between TFTs in adjacent grains in the film.
It would also be advantageous if a method could be devised of making the process of annealing polycrystalline films less dependent on the amorphous film deposition process, and variations in the process of heating the amorphous film to crystallize it.
Accordingly, in forming a polycrystalline film from a microcrystalline film, a method has been provided comprising the steps of:
a) depositing a microcrystalline film which includes microcrystallites embedded in amorphous matter; and PA1 b) annealing the film deposited in the step a) to, at least partially, form a polycrystalline film. The inclusion of embedded seed crystals in the amorphous matter encourages uniform crystal grains having a relatively large size.
In another aspect of the invention, the microcrystalline film deposited in step a) includes two thicknesses, a predetermined first thickness, and a predetermined second thickness overlying the first thickness. Step b) includes melting the second thickness of film. A controlled number of seed crystals in the first film thickness promotes crystal grains that are uniformly large size.
In another aspect of the invention, step b) includes heating the microcrystalline film deposited in step a) to selectively melt the amorphous matter, leaving a predetermined number of microcrystallites in the amorphous matter unmelted. A controlled number of seed crystals promotes crystal grains of a uniformly large size.
In another aspect of the invention, the process includes a further step, following step a), of depositing a second, completely amorphous matter, film overlying the microcrystalline film deposited in step a). The annealing in step b) includes extending crystalline regions from the microcrystalline film into the second film. The film deposition process is speeded with the use of a completely amorphous film. Preferably, the microcrystalline film has a predetermined first thickness and the second film has a predetermined second thickness. The second thickness is generally less than approximately 25% of the combined first and second thicknesses.
Step a) also includes depositing a film including amorphous matter embedded with microcrystallites having a size generally in the range between 50 .ANG. and 500 .ANG.. Control over the size and stability of crystalline clusters is responsive to the size of the seed crystals.
In one preferred embodiment, the amorphous matter and microcrystallites of the film deposited in step a) are silicon. In another aspect of the invention, the amorphous matter and microcrystallites of the film deposited in step a) are a silicon-germanium compound.
A liquid crystal display is also provided comprising a transparent substrate, and a TFT polycrystalline semiconductor film overlying the transparent substrate. The TFT polycrystalline semiconductor film is formed from depositing a microcrystalline film, including amorphous matter embedded with microcrystallites, on the substrate, and annealing the microcrystalline film to create polycrystalline TFT film. The inclusion of embedded seed crystals in the amorphous film encourages uniform crystal grains having a relatively large size.